1. Field of the Invention
The present invention relates to a liquid crystal display device, more specifically a liquid crystal device including a substrate provided with a line pattern, and a circuit element connected to the line pattern.
2. Description of the Related Art
A liquid crystal display device includes a liquid crystal display panel which includes a pair of substrates and a liquid crystal layer interposed therebetween. The liquid crystal display device further includes a circuit element such as a driver IC (Integrated Circuit) which is mounted on the liquid crystal display panel for driving the liquid crystal display panel. Conventionally, a TCP (Tape Carrier Package) technique is known as a general technique for mounting the driver IC on the liquid crystal display panel. Recently, in view of low cost, high reliability, capability of producing thin products and the like, a COG (Chip On Glass) technique is receiving attention. According to the COG technique, the driver IC for driving the liquid crystal display panel is bear-chip mounted on the liquid crystal display panel. Generally, a protrusion-like bump is formed on each electrode of the driver IC so that the driver IC is electrically connected to the liquid crystal display panel at the protrusion-like bumps in a face-down bonding manner. In this case, the bump electrodes are made of solder which are later melted for connection to the liquid crystal display panel. Alternatively, the bump electrodes are made of a metal such as Au which are later connected to the liquid crystal display panel by a conductive paste or an anisotropic conductive adhesive, e.g., an insulative adhesive having conductive particles dispersed throughout.
FIG. 9 is a cross-sectional view showing a conventional liquid crystal display device 300. FIG. 10 is a partially enlarged view of a portion 50C in FIG. 9, where an electrode pad 4 is seen toward a direction along an aligning direction thereof (hereinafter, simply referred to as "an aligning direction of the electrode pads") (the rest of the electrode pads 4 are hidden behind the electrode pad 4 as shown in FIG. 10).
Hereinafter, the conventional liquid crystal display device 300 will be described in which a liquid crystal display panel 3 and a driver IC 1 for driving the liquid crystal display panel 3 are connected in a face-down bonding manner by an anisotropic conductive adhesive 2.
Referring to FIG. 10, the plurality of electrode pads 4 (the rest of the electrode pads 4 are hidden behind the electrode pad 4 in FIG. 10) are formed on the surface of the driver IC 1. Each electrode pad 4 is provided with one protruding-like bump electrode 5. One of a pair of substrates 3a and 3b, i.e., the substrate 3a, is provided with a plurality of bonding pads 6 corresponding to the respective electrode pads 4.
FIG. 11 is a cross-sectional view showing the conventional liquid crystal display device 300 seen toward a direction indicated by an arrow 20 in FIG. 9. FIG. 12 is a partially enlarged view of one example of a portion 50D in FIG. 11, where the electrode pads 4 are seen toward a direction vertical to the aligning direction thereof.
Referring to FIG. 12, as described above, each of the plurality of electrode pads 4 and each of the plurality of bonding pad 6 are aligned so as to correspond with each other in a one-by-one manner. Each electrode pad 4 is provided with one bump electrode 5. The bump electrodes 5 and the bonding pads 6 are electrically connected to each other by an anisotropic conductive adhesive 2 which is an insulative adhesive 8 having conductive particles 7 dispersed throughout.
According to a method using the anisotropic conductive adhesive 2, the conductive particles 7 contained in the anisotropic conductive adhesive 2 are sandwiched between and make contact with the bump electrodes 5 of the driver IC 1 and the bonding pads 6 of the liquid crystal display panel 3, thereby providing electrical connection between the driver IC 1 and the liquid crystal display panel 3. This structure is advantageous, for example, in that the pitch of portions provided with electrical connection between the driver IC 1 and the liquid crystal display panel 3 depends only on the size of the bump electrodes 5, and in that the insulative adhesive 8 fills in the spaces between the adjacent bump electrodes 5 so as to advantageously provide sufficient insulation therebetween. Because of these advantages, the method using the anisotropic conductive adhesive 2 has become significant in the COG system.
In the above-described conventional liquid crystal display device, in order to realize a shorter pitch of the adjacent electrode pads 4 for the purpose of reducing the area required for mounting the driver IC 1, a shape of each electrode pad 4 is made into a rectangular shape or the like where minor sides of the electrode pads 4 are aligned along the aligning direction. Moreover, in order to realize a further shorter pitch of the adjacent electrode pads 4, the adjacent electrode pads 4 may be alternately offset in a direction vertical to the aligning direction of the electrode pads 4 so as to form a zigzag arrangement.
Referring to FIGS. 13A and 13B, the bump electrodes 5 of the conventional liquid crystal display device 300 are formed as follows. First, a photoresist 100 is applied on the driver IC 1 which is provided with the plurality of electrode pads 4. The photoresist 100 is provided so as to be about several .mu.m to about 5 .mu.m thicker than the thickness of the bump electrodes 5 to be formed. As shown in FIG. 13A, the photoresist 100 is exposed to light and developed so as to leave openings where the bump electrodes 5 are to be formed. The openings are formed so that an opening corresponds to one electrode pad 4. Then, the surface of the driver IC 1 with the photoresist 100 is immersed in a plating solution (e.g., of Au). A suitable amount of current is flowed between an electrode immersed in the plating solution and the driver IC 1, such that Au is deposited in the openings as shown in FIG. 13B. After the plating, the photoresist 100 is removed, thereby forming the bump electrode 5 on each electrode pad 4.
However, the bump electrodes 5 formed as described above have problems such as failure in growing the plating which depends on the conditions of the openings in the photoresist 100 or the conditions of the plating solution itself. This will result in loss or non-uniform heights of the bump electrodes 5. For example, the plating solution is circulated so that the openings are supplied with the plating solution. During the circulation, however, cavitation (i.e., bubbles) may be created which may block the openings and interrupt the growth (deposition) of the plating, and thereby cause a reduction in the height of the bump electrode 5. Furthermore, when a dust is present on a photomask during the light exposure of the photoresist 100, which rarely occurs, a sufficient amount of the opening may not be formed due to an insufficient exposure to light, thereby causing a loss of the bump electrode 5.
As shown in FIGS. 10 and 12, where there is no loss or non-uniform heights of the bump electrodes 5, the conductive particles 7 are sandwiched between and successfully made contact with the bump electrodes 5 and the bonding pads 6, thereby providing electrical connection therebetween.
FIG. 14 is a partial enlarged view of another example of the portion 50C in FIG. 9 where an electrode pad 4 is seen toward the aligning direction thereof (and the rest of the electrode pads 4 are hidden behind the electrode pad 4 shown in FIG. 14). FIG. 15 is a partial enlarged view of another example of the portion 50D in FIG. 11, where the electrode pads 4 are seen toward a direction vertical to the aligning direction thereof. In FIGS. 14 and 15, the conductive particles 7 are sandwiched between the bump electrodes 5 and the bonding pads 6 but do not make contact thereto, and thus, a current does not flow therebetween, thereby causing a connection failure.